Use a hyphen and singular form of the measurement.
A 3-mile journey, a 10-year period, a 12-foot drop, a four-dollar purchase.
Although "10 years' period" isn't idiomatic, you'd use the possessive otherwise, as in: She gave two weeks' notice.
This is often written erroneously as "two weeks notice," but see this: http://www.usingenglish.com/forum/as...-articles.html
With the"of" then it's plural and no apostrophe: A journey of three miles, a period of 10 years, a drop of 12 feet, a purchase of four dollars.